In Wrath, we see that Tyrael, along with at least Auriel's support, has the idea of capturing a Prime Evil to give the Angels a tactical advantage in the eternal conflict. He has one problem with this plan: Imperious, who would rather just kill any Demon at hand outright. Furthermore, Imperious seems to have little regard for the will of his brethren and sister on council. A side from what we see in Wrath, when the Council sat in judgement, ready to determine the fate of Humanity, he was perfectly fine with the idea of wiping out Humanity when he though that Tyrael was on his side, despite the fact that that would have meant a tie, with two votes for humanity and one abstaintion. This shows that Imperious holds himself somewhat above the rest of the council. Tyrael has to get around that if he wants to imprison a prime Evil.
Fast forward a bit, and lo and behold, Tyrael has another opportunity to imprison a Prime Evil. All three of them in fact. By giving the Soulstones to the Horadrim, Tyrael gets to both aid humanity, whom he has a soft spot for, and, he gets to have his eternity long dream of having prime Evils imprisoned.
However, in doing this, he's doubly getting on Imperiouses bad side. There are two issues where the two of them have disagreed that we've been shown, and Tyrael just went behind Imperious' back with both of them. Given both Imperious' ego and how well the whole Soulstone plan went for the Angels, is it any wonder at all that Justice is falling on the World of Men?
When you take this into account that this division was started by Diablo in Wrath, and that the whle Soulstone Fiasco was part of a master plan on the part of the Prime Evils, and we've got one heck of an evil plan going on here.